Geology Reference
In-Depth Information
commonly 2 - 10 mm in diameter. They may consist of a relatively thin shell
of fine-grained ash around a core of coarse ash, or be composed wholly of
concentric layers of fine-grained ash (as in oolites). Such lapilli are believed
to be formed by accretion around solid nuclei in a medium of condensed
moisture (cloud and/or rain) within an eruption column.
2. Impact structures, commonly termed bomb-sags (Figure 5.11) in fall and flow
deposits, which result from fall of bombs (or more commonly, blocks) into
finely bedded, commonly water-saturated sediments. Such impact structures
are characteristic of pyroclastic deposits erupted in wet conditions.
Table 5.1
Grain size limits for pyroclastic fragments.
Grain size
1
Pyroclastic fragments
(
ϕ
)
(mm)
Blocks and bombs
2
- 8.0
256
Coarse
- 6.0
64
Fine
Lapilli
-1.0
2
Coarse
Ash
+
4.0
1/16
Fine
1
The grain size is expressed in two scales. The 'phi' (
) scale has values which decrease
arithmetically as grain size (expressed in mm) increases geometrically. The conversion from
the mm scale to the phi scale is arithmetically very simple and is expressed by the equation
N
=
ϕ
2
−
ϕ
,where
3, and so on. and
N
is the grain size in mm.
For example, consider a grain size of 2 mm:
ϕ
=
1
,
2
,
2
−
ϕ
2
2
−
ϕ
N
=
=
and
2
ϕ
2
=
1
/
Therefore
2
ϕ
=
1
2
=
2
−
1
So, for a grain size of 2 mm,
ϕ
=−
1.
Therefore, zero on the phi scale corresponds to a grain size of 1 mm, whereas
ϕ
=+
1
,
+
2
1
2
,
1
4
and
1
8
mm and
and
+
3 correspond, respectively, to
ϕ
=−
1,
−
2and
−
3 correspond,
respectively, to 2, 4 and 8 mm, and so on.
2
A block that takes on a distinctive form during fragmentation, flight or upon landing is termed
a volcanic bomb. Such bombs are often given descriptive names such as ribbon bombs, spindle
bombs, breadcrust bombs and cowdung bombs.
Sorting (
σ
I)
: (Inclusive Graphic Standard Deviation)
(φ
84
−
φ
16
)
4
+
(φ
95
−
φ
5
)
6
6
φ
x
is the grain
size (
phi
units) at the
x
% value on the cumulative frequency curve (see Tucker, this topic
series).
σ
I
=
.
